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Re: XPath and find/findAll methods

From: Jonas Sicking <jonas@sicking.cc>
Date: Tue, 22 Nov 2011 16:08:41 -0800
Message-ID: <CA+c2ei9H0HFEef_+NREovbiHQpEmxFQexTUXOrpj91yEeqvT1Q@mail.gmail.com>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
Cc: Robin Berjon <robin@berjon.com>, Boris Zbarsky <bzbarsky@mit.edu>, public-webapps@w3.org
On Tue, Nov 22, 2011 at 11:52 AM, Tab Atkins Jr. <jackalmage@gmail.com> wrote:
> On Tue, Nov 22, 2011 at 11:38 AM, Robin Berjon <robin@berjon.com> wrote:
>> On Nov 22, 2011, at 18:31 , Boris Zbarsky wrote:
>>> On 11/22/11 12:29 PM, Robin Berjon wrote:
>>>>>> d - "//div[parent::*//a]";
>>>>>
>>>>> (d) can be done with the new subject indicator in the Selectors 4
>>>>> draft:   a!>  div  (syntax pending, but that's the general idea)
>>>>
>>>> I think that the example you show selects the<a>  parent of a<div>, not<div>s that have parents containing an<a>.
>>>
>>> Yes, that's what Tab's selector selects too.  The '!' after the 'a' is important there.
>>
>> Wait, I thought I'd grasped the gist of S4 but now you're confusing me :) My reading of Tab's selector is that it
>> matches the <a> in <a><div/></a>. What Martin's XPath matches is the <div> in <section><div/><p><a/></p></section> (amongst many other variants). It's "all div's whose parents have an a descendent".
>
> Pretty sure that Boris misunderstood your comment.  I did too, at
> first, since I wasn't sure which example you were referring to.  ^_^
>
> You are correct on both counts.  "a! > div" matches <a> elements with
> a <div> child.  To match the XPath selector, you need something more
> like "*:matches(:scope a) > div", which isn't yet a valid selector,
> but is roughly in line with how we want to extend :matches() in the
> future.

I really don't think that selectors can ever compete with the
expressiveness of XPath. Consider the following expression:

//div[count(.//span) > 6][count(.//span[@data-foo = ../@data-bar]) mod 2 = 1]

This expression finds all <div> elements which has at least 6 <span>
descendants and where an odd number of those <span> elements have a
"data-foo" attribute equal to its parents "data-bar" attribute. It is
obviously trivial to add arbitrary additional complexity to this
expression.

Trying to do the same thing in Selectors will just result in a
incomprehensible mess.

At the same time, XPath can't ever compete in expressiveness to
Javascript. Finding all <div> elements with a "data-foo" attribute
that contains a prime number is not possible in XPath but trivial in
javascript.

I'm not convinced that it's worth investing in XPath. At least not
beyond the low-hanging fruit of making most of the arguments to
.evaluate optional. But I think trying to make selectors compete in
expressiveness with XPath is a loosing battle.

/ Jonas
Received on Wednesday, 23 November 2011 00:09:41 GMT

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